The present invention relates to the art of magnetic resonance imaging (MRI). It finds particular application in conjunction with fast spin echo (FSE) imaging sequences, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other magnetic resonance spectroscopy and imaging applications, particularly MRI techniques employing multi-echo pulse sequences having relatively long echo-trains such as, e.g., Sliding INterleaved ky (SLINKY) acquisition techniques, shifted interleaved multi-volume acquisition (SIMVA) techniques, other three-dimensional fast spin echo (3DFSE) techniques, etc.
Commonly, in MRI, a substantially uniform temporally constant main magnetic field, B0, is set up in an examination region in which a subject being imaged is placed. Via magnetic resonance RF excitation and manipulations, selected magnetic dipoles in the subject which are otherwise aligned with the main magnetic field are tipped (via radio frequency (RF) pulses) into a plane transverse to the main magnetic field such that they precess or resonate. In turn, the resonating dipoles are allowed to decay or realign with the main magnetic field thereby inducing magnetic resonance echoes. The various echoes making up the MRI signal a encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI apparatus is collected into a matrix commonly known as k-space. Typically, each echo is sampled a plurality of times to generate a data line or row of data points in k-space. The echo or data lines position in k-space is determined by its gradient encoding. Ultimately, employing Inverse Fourier or other known transformations, an image representation of the subject is reconstructed from the k-space data.
At times, due to non-ideal system performance or in the case of some specific data acquisition strategies, MRI signals are distorted or contaminated in either phase or amplitude leading to data inconsistencies in k-space. One potential inconsistency is that each resonance excitation is not precisely the same amplitude or the same phase. Consequently, the result is degraded image quality. Traditional methods of addressing this problem are designed to minimize those known factors affecting image quality, such as gradient non-linearity, etc. One of these methods is, for example, pre-calibration in FSE or multi-shot type sequences which pre-emphasize the data error. Typically, resonance is excited and all or a portion of an echo train is generated to produce reference or calibration echoes. However, with longer echo-trains, such techniques are relatively time consuming and inefficient. Moreover, as the relevant techniques only use data collected prior to the imaging experiment, they are limited to correcting for k-space data errors that result from anomalies which are time-invariant over the duration of the imaging sequence (for example, spatially dependent errors). Accordingly, time dependent error such as, e.g., those created by motion or other dynamic factors, are not compensated for.
The present invention contemplates a new and improved technique for addressing data inconsistencies in k-space which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance imaging includes producing a plurality of imaging sequences with an MRI apparatus. Each imaging sequence generates a multiple echo echo-train which issues from a subject being imaged. Each echo is phase encoded and collected into k-space as a plurality of sampled data points. The plurality of sampled data points are located in k-space based upon the phase encoding of each collected echo. An additional navigator echo is generated and collected in conjunction with each of the plurality of imaging sequences. The sampled data points in k-space are adjusted to account for data inconsistencies based upon information gleaned from the collected navigator echoes.
In accordance with another aspect of the present invention, a magnetic resonance imaging apparatus includes a main magnet that generates a substantially uniform temporally constant main magnetic field through an examination region wherein an object being imaged is positioned. A magnetic gradient generator produces magnetic gradients in the main magnetic field across the examination region. A transmission system includes an RF transmitter that drives an RF coil which is proximate to the examination region. A sequence control manipulates the magnetic gradient generator and the transmission system to produce a plurality MRI pulse sequences. Each of the MRI pulse sequences induces (i) an echo-train including a number of imaging echoes and (ii) a navigator echo, both of which stem from the object. A reception system includes a receiver that receives the imaging echoes and the navigator echoes. The imaging echoes get loaded into a first data storage device as k-space data and the navigator echoes get loaded into a second data storage device. A data processor accesses the first and second data storage devices to correct the k-space data in accordance with error factors generated from the navigator echoes. A reconstruction processor accesses the first data storage device and applies a multi-dimensional Fourier transformation to the k-space data therein such that an image of the object is reconstructed. Ultimately, an output device produces a human viewable rendering of the image.
One advantage of the present invention is improved image quality and artifact elimination.
Another advantage of the present invention is efficient data acquisition.
Yet another advantage of the present invention is the ability to correct for time-variant data errors in k-space.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.